A mixer is a device that converts signals from one frequency to another. This is accomplished by applying a local oscillator (LO) reference signal to one of the mixer ports, and the signal to be converted to a second mixer port. The sum and difference frequencies of the two signals are predominantly what appear at the mixer output port, although the sum and difference frequencies of the harmonics of the two input frequencies also co-exist on the mixer output port as well. For transmitter applications, a low frequency signal is first processed (or modulated) and then up converted through a mixer to a microwave frequency, where it is usually amplified and transmitted. For receiver applications, the received signal is down converted through a mixer to a low frequency where the information is extracted (demodulated).
The diode, a non-linear device, creates the mixing action in a mixer. Diodes, the simplest kind of semiconductor, consisting simply of a package with two leads, permit current flow in one direction only. Like any semi-conductor, a diode is capable of handling only a limited amount of input signal power before it saturates.
A mixer's dynamic range is the input power range over which a mixer is usable. On the low input power end, it is limited by the noise figure and other system parameters such as signal to-noise ratio and receiver bandwidth. On the high end it is limited by either the saturation level or the input level for which certain spurious signals reach unacceptable levels.
To cover a broad range of frequencies, combinations of mixers have been used in order to get adequate coverage. This results in disadvantageous bulk and cost. Instrumentation design reflect a compelling tendency toward remote instrument heads. There is a need for a mixer capable of operating across broad RF and local oscillator frequency bandwidths to meet the design and performance requirements .